Coupling losses usually occur at junctions between optical fiber trunks and are chiefly due to transverse and/or longitudinal offsets between the two trunks.
A method known in the art of measuring coupling losses is described in the paper entitled: "Direct method of determining equivalent-step-index profiles for monomode fibers", C. A. Millar, Electronics Letters, 25th June 1981, Vol. 17, No. 13.
According to this method, two lengths of the same single-mode optical fiber are placed facing each other with the axes aligned, and an optical beam is injected into them.
A transverse offset is then introduced between the two fibers which allows the measurement of the optical power transferred from one fiber to the other, and consequently of the amount of the coupling losses.
The method allows the measurement of the Mode Field Diameter MFD which is a parameter indicative of micro-bending and coupling losses in single-mode optical fibers.
Mode Field Diameter is given by twice the transverse offset value for which the measured optical power drops to 1/e times the maximum value.
According to this method a longitudinal offset between the two fibers can be also introduced; thus also coupling losses between two fiber trunks, due to their longitudinal separation, can be measured.
The two fibers are held facing each other by means of a high precision calibrated mechanical device or mechanism comprising micromanipulators for the micrometric regulation of fiber alignment and allowing transverse and/or longitudinal offsets of desired and measurable amounts to be set.
For the maximum optical power transfer, the end faces of the two fibers should be kept in contact; this, however, would no longer allow the transverse offset regulation because of rubbing between the two end faces, which hence are kept at a distance as little as possible from each other; this distance is however, conspicuous (about 5 .mu.m).
The two fibers are simply obtained by cutting the same fiber length at a certain point; to obtain the maximum optical power transfer the two end faces ought to be planar, non-corrugated and perpendicular to the fiber axis.
However, such a cut is rather difficult to obtain; actually the two end faces are slightly oblique (with angles even wider than 1.degree. with respect to perpendicularity to the axis), corrugated and irregular, with convexities, swells and hackles.
The presence of hackles requires an increase in the distance between the two end faces to prevent their rubbing. Hence additional optical power losses are introduced due both to the unwanted longitudinal offset between the two trunks, and to the non-perpendicularity and irregularity of the end faces.
This effect alters the measurement reducing its accuracy.
Besides, by this method, a significant MFD measurement cannot be obtained when the fiber core section has a two-fold symmetrical shape (i.e. elliptical shape) instead of circular shape.
In fact the two fibers, as a consequence of the cut, usually undergo a reciprocal rotation difficult to be compensated for, which causes in this case a mismatch of the two sections and an uncertainty in the measurement of MFD value due to additional optical power losses which vary with the mutual rotation amount and with the direction and sense of the transverse offset.